I've seen this problem with an older touring bike. The bike had very wide rims, but the owner had tried to put 25mm tires on the bike. The tires were too narrow for the rim, so they didn't seat correctly, leading to a slight "bulge" in a portion of the tire.
What kind of wheel and tire? If your tire is too narrow for the rim, well, get a properly sized tire. If the tire just isn't seated correctly, do as others have said and let the air out and reseat it on the rim.